The regular telephone voice channel has been used for digital data transmission since the early 1960's. The digital data to be transmitted is modulated onto a sine wave carrier signal whose frequency is within the voice transmission band. The digital data is demodulated off the carrier signal after transmission via the telephone channel. The device used at the transmitting end to modulate the data onto the carrier is known as a modem. A modem is also used at the receiving end to demodulate the data from the carrier. Three basic modulation techniques used for the transmission of digital data over the telephone voice channel are Amplitude Modulation, Frequency Shift Keying, and Phase Shift Keying.
A medium speed full duplex modem usually allocates one-half of the voice transmission band for transmitting and the other half for receiving on a two-wire telephone channel. This is accomplished through use of a frequency division multiplexing technique wherein a transmitting subband and a receiving subband are frequency multiplexed. The highest transmission speed based on such a scheme without using data compression is around 1200 bits per second.
To further increase the transmission rate of a full duplex modem, use of an adaptive echo canceller may be required. An adaptive echo canceller can cancel echoes induced at the local loop of the telephone network due to four-wire to two-wire conversion. The adaptive echo canceller permits the entire voice band can be used for transmitting and receiving at the same time without the use of frequency multiplexing.
More particularly, at the near end of a voice channel used for data transmission, the transmitter and receiver of a modem are each connected by a two-wire pair (i.e. a total of four wires) to a hybrid. The hybrid interfaces the four-wire loop to the two-wire local loop of the telephone network. A strong echo of the transmitted data signal is induced by the four-wire to two-wire conversion performed by the hybrid. The echo arises as a result of an impedance mismatch at the hybrid so that a fraction of a data signal generated by the transmitter is not transferred to the two-wire local loop, but is instead, returned to the receiver. Other, weak echoes may also be introduced at other locations along the voice channel where four-wire to two-wire conversions take place.
Thus, the signal received at a receiver of a modem includes an actual data signal to be received and an echo component. It is the role of the echo canceller to reduce the echo component of the received signal as much as possible so that the actual data signal to be received is reliably detected.
In a conventional adaptive echo canceller, the echo path is approximated by an adaptive digital filter whose impulse response approximates as closely as possible the impulse response of the actual echo path. As the transmitted data is available to the echo canceller, the transmitted data is operated on by the estimated echo path impulse response as provided by the echo canceller filter to obtain an estimated echo signal. The estimated echo signal is then subtracted from the received signal to approximately eliminate the echo component from the received signal. Thus, the echo canceller is driven by the transmitted data. The data driven echo canceller has been developed for the purpose of true full duplex (i.e. no frequency division multiplexing and no time division multiplexing) data transmission over the voice telephone channel.
The algorithm used most often to estimate the echo path impulse response is the Least Mean Square (LMS) algorithm. Based on the error between transmitted training data and a received echo signal, the LMS algorithm iteratively converges to set up filter coefficients for an adaptive digital filter whose impulse response estimates the actual echo path impulse response. The LMS algorithm then continually adjusts the filter coefficients which characterize the estimated echo path impulse response so as to make the error between the actual echo signal and the estimated echo signal as small as possible.
As the requirements for echo reduction increase, the effects of nonlinearities in the echo path come into play. A conventional adaptive echo canceller utilizing an LMS algorithm can only cancel those portions of an echo signal resulting from the linear component of the echo path. Nonlinear echo components cannot be cancelled by the conventional LMS data driven echo canceller.
The effects of nonlinear echo components are most severe for higher bit rate transmissions at many different signal levels as in the ISDN (Integrated Services Digital Network) local subscriber loop.
A variety of approaches for handling nonlinear echo components have been proposed. A general approach for nonlinear echo cancellation was suggested in E. J. Thomas, "Some Considerations on the Application of the volterra Representation of Nonlinear Networks to Adaptive Echo Cancellers," B.S.T.J., Vol. 50, No. 8, pp. 2797-2805, Oct. 1971 where the nonlinear component of the echo signal was characterized by a Volterra integral equation. In particular, a nonlinear echo canceller with many extra sets of taps corresponding to the nonlinear integration terms is disclosed. In N. H. Holte and S. Stueflotten, "A New Digital Echo Canceller for Two-Wire Subscriber Lines," IEEE Trans. Commun., Vol. COM-29, No. 11, pp. 1573-1580, Nov. 1981, a memory compensation based technique for nonlinear echo canceller is proposed. In O. Agazzi, D. G. Messerschmitt, and D. A. Hodges, "Nonlinear Echo Cancellation of Data Signals," IEEE Trans. Commun., Vol. COM-30, pp. 2421-2433, Nov. 1982, a special binary series expansion technique is disclosed, and most recently, in M. J. Smith, C. F. N. Cowan, and P. Adams, "Nonlinear Echo Cancellers Based on Transpose Distributed Arithmetic," IEEE Trans. Accoust., Speech, Signal Processing, Vol. ASSP-35, pp. 6-17, Jan. 1988, a combined linear and nonlinear structure is proposed.
However, all of the above-mentioned approaches for eliminating the nonlinear components of an echo require extra computation power in comparison to the conventional linear LMS echo cancellation technique. In addition, the above-mentioned prior art techniques for cancelling the nonlinear component of an echo exhibit slow convergence.
In view of the foregoing, it is an object of the present invention to provide a nonlinear data driven echo canceller which overcomes the problems of the above-identified prior art nonlinear echo cancellers. More particularly, it is an object of the present invention to provide a nonlinear data driven echo canceller which has a simple structure, which requires a relatively small amount of computational power, and which converges relatively rapidly.